Recently, along with enhancement of the operating speed of personal computers, the spread of network infrastructures, and increased and lower-priced mass storage, data of documents or image, which were conventionally printed on paper, can be received simply as electronic information so that opportunities to read such electronic information have increased dramatically.
There were used, as a means for reading electronic information, conventional liquid crystal displays or CRT and recent emission type displays, such as organic electroluminescence displays. Specifically, when electronic data is document data, it is necessary to notice this reading means over a relatively long period of time. It is hard to say that such an action is a kindly means to humans. There are generally known disadvantages of emission type displays such that flickering tires human eyes, they are awkward to carry about, the reading posture is restricted, it is necessitated to gaze at a stationary picture plane, and electric power consumption increases when reading over a long time.
As a display means to redeem the foregoing disadvantages is known a reflection display having memory capability which employs external light and does not consume power for image retention. However, based on the reasons below, it is hard to say that such a display provides sufficient performance.
For instance, a system using a polarizing plate such as a reflection type liquid crystal display exhibits a relatively low reflectance up to 40%, resulting in difficulty in displaying whiteness and methods of preparing constituent members are not necessarily simple. A polymer dispersion liquid crystal display requires a relatively high voltage and employment of the difference in refractive index between organic compounds does not result in images with sufficient contrast. A polymer network type liquid crystal display has problems such that it requires a relatively high voltage and a complex TFT circuit to enhance memory. An electrophoretic display element needs relatively high voltage of more than 10 V, and there is concern of durability of the electrophoretic particles, due to their tendency to coagulate. An electrochromic display element, which can be driven at a relatively low voltage of not more than 3 V, has problems that it is insufficient in color quality of black or colors (such as yellow, magenta, cyan, blue, green and red) and its display cells require complex layer arrangement such as a deposit layer to maintain memory.
There is known, as a display system to overcome these disadvantages of the foregoing systems, an electro-deposition (hereinafter, also denoted simply as ED) system which employs dissolution-deposition of metals or metal salts. The ED system, which can be driven at a relatively low voltage of not more than 3 V, exhibits advantages such as simple cell constitution and being superior in black and white contrast and in black color quality. There were disclosed various methods (as described in, for example, Patent documents 1-3).
As a result of the inventor's detailed study of the technique disclosed in the foregoing patent documents, it was proved that in conventional techniques, there were produced problems such that repeatedly driving resulted in variation in color in the midtone.                Patent document: U.S. Pat. No. 4,240,716        Patent document 2: Japanese Patent No. 3428603        Patent document 3: JP-A No. 2003-241227 (hereinafter, JP-A refers to Japanese Patent Application publication)        